Boiler Tube Failure due to Opening a Drain Valve

We had just erected and commissioned a circulating fluidized bed boiler for an industrial plant in Southeast Asia. The boiler was a state of the art CFBC and almost the largest boiler in the region at that time. It was capable of producing nearly 400 tons per hour of steam for power generation and utility purposes, firing coal and biomass with superheated steam at a pressure of 140 bar and temperature of 540°C.



A basic understanding of a boiler is required to understand what went wrong
Furnace
The furnace of a modern boiler is made of welded tubes called membrane panels. The carbon steel tubes of diameter 60.3 mm welded at a pitch of 75 mm forms the boiler furnace with a height of 40 meters, width of 12 meters, and depth of 8 meters. The heat transfer area was around 1700 square meters. The heat in the furnace transfers to the water in these tubes to produce steam. The furnace is at temperatures in the range of 800°C to 1000 °C. The water flowing through the tubes keeps the tube metal temperatures well below the deformation temperatures. If there is no water flow in these tubes, the tubes will overheat and fail. This is true for any vessel that boils water. Even in your house, if you heat a pot or a pan without water it will twist out of shape.